(1) Field of the Invention
The present invention relates to an electro-oculography apparatus that removes or detects a signal generated by an eye blink and further detects a saccade signal in an eye potential signal of a user with ease and high accuracy.
(2) Description of the Related Art
Methods for detecting an eye movement includes: an electro-oculography (EOG) that utilizes a potential generated between a cornea and a retina; a corneal reflex method that detects a movement of a virtual image generated on a cornea by irradiating a spotlight on an eyeball; a strong-reflection method that uses a difference in reflectance between the cornea and the retina; a method using contact lenses; and so on.
Here, the EOG is a method for detecting an eye movement that utilizes the fact that a human cornea is charged positively with respect to the retina. More specifically, electrodes are placed near a human eyeball and a change in a potential measured by the electrodes is used to detect the eye movement.
FIG. 14A and FIG. 14B show examples of the method for detecting an eye movement which employs the EOG. FIG. 14A shows an example of the case where electrodes A and B are placed to the outside and the inside of the right eye of a user, the same distance away from the center of the eyeball, in which the outer electrode is A and the inner electrode is B. Assuming the eye potential occurring on the electrode A is Va and the eye potential occurring on the electrode B is Vb, Va and Vb are equal when the eyeball of the user is positioned in the center as in FIG. 14A, and the eye potential Va-b measured is 0V.
On the other hand, when the user looks to the right as in FIG. 14B, the electrode A becomes closer to the cornea of the right eye, and thus Va becomes greater than Vb and the measured eye potential Va-b indicates a positive value. Conversely, when the user looks to the left, Va becomes smaller than Vb and the measured eye potential Va-b indicates a negative value. In other words, when the measured eye potential Va-b shows a positive value, it is indicated that the user has moved his or her eye to the right, and when the measured eye potential Va-b shows a negative value, it is indicated that the user has moved the eye to the left. In the EOG method, such changes in the measured eye potential Va-b as described above are utilized, so that an eye movement of a user is detected.
When detecting an eye movement by utilizing a change in an eye potential as in the EOG and the like, there is a problem of an effect of a signal generated by a blink of a user (hereinafter referred to as “blink signal”).
In some cases, the blink signal is generated invariably in the positive direction, or invariably in the negative direction, depending on the method for measuring the eye potential.
FIG. 15A to FIG. 15D show examples of patterns of placing the electrodes and the methods for measuring the blink signal. According to the placement pattern of FIG. 15A, the electrodes A and B are placed above and below an eye, respectively, and a difference potential Va-Vb is obtained, where Va is the eye potential measured by the electrode A placed above the eye and Vb is the eye potential measured by the electrode B placed below the eye. In this case, the blink signal is generated invariably in the positive direction. This is because, when a human blinks, the eyeball always moves upward.
According to the placement pattern of FIG. 15B, the electrode A is placed above the eye and the other electrode is placed on the earth or a place less subject to the eye potential, so that the eye potential Va of the electrode A is measured. In this case also, the blink signal is generated invariably in the positive direction (at a value larger than a reference value).
Likewise, according to the placement pattern of FIG. 15C, the electrodes A and B are placed above and below the eye, respectively, and a difference potential Vb-Va is obtained, where Vb is the eye potential measured by the electrode B placed below the eye and Va is the eye potential measured by the electrode A placed above the eye. In this case, the blink signal is generated invariably in the negative direction. According to the placement pattern of FIG. 15D, the electrode B is placed below the eye and the other electrode is placed on the earth or a place less subject to the eye potential, so that the eye potential Vb of the electrode B is measured. In this case also, the blink signal is generated invariably in the negative direction.
When the user blinks during the measurement of the eye potential according to the placement patterns as shown in FIG. 15A and FIG. 15B, a potential is generated steeply in the positive direction (this is the “blink signal”) as shown by regions (a) in FIG. 16. When the signal is treated directly as an eye-gaze movement, a gaze-point changes rapidly and a gaze-path cannot be tracked accurately.
Here, there is a technique disclosed by Japanese Unexamined Patent Application Publication No. 11-85384 (Patent Reference 1) as a method to reduce an effect of the blink signal (a component of a signal generated by a blink) and the like from eye potential original signal.
The technique disclosed by Patent Reference 1 aims to detect an eye potential of a user and input a gaze-position (cursor) in real time. At this time, a delay element is introduced into a fluctuation waveform of the eye potential, so that a temporal change in the gaze-position (cursor) is smoothed and a rapid change in the gaze-position caused by a blink is reduced.
Further, there is a technique disclosed by “Full-time Wearable Headphone-Type Gaze Detector”, Interaction 2006, pages 23 to 24, 2006 (Non-Patent Reference 1), Hiroyuki Manabe, Masaaki Fukumoto, as a technique reducing an effect of the blink signal.
According to the technique disclosed in the Non-Patent reference 1, a total of 8 electrodes are placed on the right and left of a headphone. A median filter is applied at 0.4 second intervals as to changes in the eye potential obtained from the 8 electrodes, thereby removing a change caused by a blink signal that is shorter than the above-described time interval.
However, as shown in the Patent Reference 1, merely temporally smoothing the eye potential original signal causes an adverse effect that the smoothing is performed even on a saccade waveform indicating a component change in a saccade (a rapid movement of a human eye from one gaze-point to another gaze-point (saccadic movement)) that is important in tracking a gaze-path.
Here, a saccade (saccadic eye movement) is an eye movement that occurs due to capturing an object projected on a peripheral retina where resolution is low, at a central fovea of retina where resolution is high. It is known that the speed is significantly high, at 100 to 500 (°/sec). In FIG. 16, the saccade signal is shown in portions indicated as regions (b), in which a potential changes rapidly, retains the level for a fixed amount of time (fixation), and then returns to the original potential level. This is an example of the case where an eyeball is moved in saccade from a target A to a target B, and then moved again in saccade from the target B to the target B. In general, a human obtains information on surroundings by repeating fixation for approximately 0.3 seconds and saccade for several dozen milliseconds.
When a median filter is applied to the eye potential original signal as shown in the Non-Patent reference 1, although blink signal that has been generated singly can be removed as shown in FIG. 17, the effects of blink signals that have been generated continuously for at least a predetermined amount of time cannot be completely removed. In addition, an adverse effect that a part of the saccade waveform breaks is caused.
Therefore, the above-described references have not made it clear what smoothing filter should be applied how long and in what order is optimum, in consideration of removal of blink signal and retaining saccade signal.